Radio communication systems rely on modulating carrier frequencies (i.e. a channel) in a finite portion of the electromagnetic spectrum to wirelessly transmit and receive signals. Modulation can be performed on the amplitude, frequency, and/or phase of the carrier frequency to separate the signal from unwanted noise. The signals typically convey information such as voice, video, and computer data between transmitting/receiving devices such as voice terminals (e.g., wireless phone) and data terminals (e.g., portable computer).
In order to transmit the signals over a large distance, a relay such as a satellite may be used. Passive communication satellites may be used to return transmitted signals to earth on the same carrier frequency they were transmitted on. Active communication satellites can be used to receive the transmitted signals on one carrier frequency and to retransmit the signals on another carrier frequency. Geosynchronous satellites are especially well suited for such a task due to their stationary position relative to the earth's surface.
Often it is desirable to communicate in a netted broadcast fashion. For example, a military commander may need to transmit an order to a large number of receivers which individually confirm that they received the order, or a data server may need to transmit information to multiple clients who verify reception. Currently, systems relay broadcast signals on a particular frequency and people who want to participate, tune their receiver to that channel, as in a UHF satellite system. Alternatively, conference calls may be established. Each participant in the conference call communicates with a central station such as a cell site or satellite using a pair of unique carrier frequencies. The central station combines the signals of conferencing callers and then transmits the combined signal to each receiver using its unique carrier frequency.
Prior art approaches for netted broadcast communication suffer from certain drawbacks. For instance, conference calling requires one channel per user. As the number of receivers grows the channels-are exhausted. Therefore, only a relatively small number of receivers may participate. Similarly, UHF and other broad beam systems have a relatively small number of channels and require users to tune their receiver to one particular channel. None of these techniques are well suited for use in a digital beam forming satellite communication system, wherein multiple beams distinguished by frequency band are used to cover multiple geographical regions while reusing frequencies to increase capacity in a spectrum limited system.